In a photoresist process which is one of processes of manufacturing a semiconductor device, a resist solution is applied to the surface of a semiconductor wafer to form a resist film, and the resist film is exposed for the transfer of a predetermined pattern and thereafter is developed, thereby to form a resist pattern. A series of these processes are generally performed using a system comprising a coating and developing apparatus for the application of resist and development and an exposure apparatus connected thereto.
To meet the recent demand for miniturization of device patterns, there has been proposed an exposure technique called immersion exposure for the purpose of enhancing the resolution of exposure. The immersion exposure is an exposure technique wherein, in a state in which a light transmitting liquid layer such as a ultra-pure water layer is formed on the surface of a wafer, light emitted from a light source is allowed to pass through the liquid layer and to irradiate the wafer surface with the light. The immersion exposure attains exposure of a high resolution by utilizing the effect that the wavelength of light in water becomes shorter than that in the air. For example, in a case where an exposure light source is ArF, the wavelength of ArF light in the air is 183 nm, while it is 134 nm in water. The immersion exposure technique is described in Japanese Patent Laid-Open Publication JP2005-294520A (the corresponding European patent laid-open publication: EP1732108A1), for example.
With reference to FIGS. 14 to 17, a description will now be given about an immersion exposure process and related problems. Before being loaded into a coating and developing apparatus, thin films of various materials (not shown for simplification of the drawings) are already formed on a silicon wafer W to be subjected to immersion exposure. In the coating and developing apparatus, an antireflection film 15, a resist film 16 and a protective film 17 are further formed on the wafer W in that order. The protective film 17 is a light-transmitting, water-repellant, thin film formed of an organic material, especially a fluorine-based material to protect the resist film 16 from the liquid used in immersion exposure.
When loaded into an exposure apparatus, the wafer W is placed on an exposure stage (not shown) and is held in a horizontal attitude. An immersion exposure head 1 is disposed above the wafer W through a slight gap between it and the wafer. A lens 10 is disposed the center of the front end of the immersion exposure head 1. Provided outside the lens 10 are a supply port 11 for supplying a liquid, e.g., pure water to the surface of the wafer W and a suction port 12 for sucking and recovering the pure water supplied to the wafer W. By supplying pure water to the surface of the wafer W from the supply port 11 and recovering the pure water from the suction port 12, a liquid layer 18 of pure water is formed between the lens 10 and the surface of the wafer W. Light emitted from a light source (not shown) passes through both lens 10 and liquid layer 18 to irradiate the wafer W with the light, whereby a predetermined circuit pattern is transferred to the resist film 16.
Subsequently, with the liquid layer 18 left present between the lens 10 and the surface of the wafer W, as shown in FIG. 15, the immersion exposure head 1 is moved to a position corresponding to the next transfer region (shot region) and the next exposure is performed. By repeating this operation, circuit patterns are transferred one after another onto the surface of the wafer W.
As known, a device is not formed in the peripheral edge area of the wafer and the outermost peripheral portion in the peripheral edge area is a beveled portion (slant portion). For the purpose of preventing the generation of particles or the like, the peripheral edge portions of the antireflection film 15 and that of the resist film 16 are removed by cleaning processes using a solvent after formation of both films, respectively. Consequently, as shown in FIG. 16, a step may be formed between the peripheral edge of the antireflection film 15 and that of the resist film 16. Also, as the foregoing various thin films (not shown) formed on the wafer W before being loaded into the coating and developing apparatus have been subjected to peripheral edge removing processes after film forming processes, respectively, the peripheral edge portions of those various thin films are also in a similar state to the antireflection film 15 and resist film 16. Accordingly, after the peripheral edge removing process for the resist film 16, the surface of the wafer W, i.e., a silicon surface 1A, is exposed in the peripheral edge area of the wafer. As a result, there exist portions where the protective film 17 is formed directly on the silicon surface 1A.
Since the protective film 17 is formed of an organic material as mentioned above, the adhesion of the protective film 17 to the silicon surface 1A is lower than that to the resist film formed of an organic material. Since the immersion exposure head 1 moves in some cases at a high speed of about 500 mm/sec during exposure, when the immersion exposure head 1 moves from the peripheral edge portion of the wafer W toward the central part together with the liquid layer 18, the protective film 17 may possibly peel off from the silicon surface 1A due to pressure from the liquid layer 18, as shown in FIG. 16(b). Moreover, in the stepped portion between the antireflection film 15 and the resist film 16, the adhesion of the protective film 17 may be low due to the complicated surface shape of the portion. Also in this case, due to movement of the immersion exposure head 1 together with the liquid layer 18, the protective film 17 may peel off near the stepped portion, as shown in FIG. 16(c). As a result, the edge portion of the resist film 16 may be exposed without being covered with the protective film 17 and peel off under the pressure of pure water which results from the movement of the immersion exposure head 1 together with the liquid layer 18. In this case, it may be impossible to carry out a normal developing process.
As shown in FIGS. 16(b) and 16(c), if the protective film 17 and the resist film 16 peel off from the wafer W, the peeled resist film 16 and protective film 17 become particles 19. The particles 19 may adhere to the lens 10 of the exposure head 1 to obstruct a normal exposure process, or may again adhere to the wafer W to adversely affect the results of various treatments after exposure of the wafer W. The particles 19 may be scattered onto an exposure stage for placing the wafer W thereon, adhere to the succeeding wafers W, and adversely affect the treatment for those wafers. Such problems and solutions thereto are not described in JP 2005-294520A.
In some cases, in order to improve the adhesion of the resist film 16 to the surface of the wafer W, HMDS (hexamethyldisilazane) gas is supplied to the whole surface of the wafer W after formation of the antireflection film 15 and before formation of the resist film 16 to perform a hydrophobizing process (AD process) to the surface of the wafer W. This hydrophobizing process may possibly enhance the adhesion of the protective film 17 to the silicon surface 1A. However, if the formation of the resist film 16 and the removal of the resist film 16 in the peripheral edge portion of the wafer are performed after the hydrophobizing process, the effect of the hydrophobizing process on the silicon surface 1A is considerably deteriorated. Thus, it may be impossible to ensure sufficient adhesion between the silicon surface 1A and the protective film 17 even if such a hydrophobizing process is performed.